Optical signal processing device and non-linear optical component

ABSTRACT

An optical signal processing device equipped with a source of electromagnetic radiation of variable intensity, a non-linear optical component, which comprises at least one photoluminescent carbon nanotube, and with a means of detecting electromagnetic radiation utilizes the non-linearity of the photoluminescence of carbon nanotubes for optical signal processing. The invention also relates to a non-linear optical component.

The invention relates to an optical signal processing device, whichcomprises a source of electromagnetic radiation, a non-linear opticalcomponent and a means of detecting electromagnetic radiation and whichcan be used as photonic component, sensor, optical switch, opticaltransistor, optical amplifier, optical memory and as optical logicelement for an optical computer. The areas of application lie in opticalinformation transmission, sensor systems and integrated non-linearoptics.

Non-linear optical components and non-linear optics can be used toconstruct digital optical memories and logic gates [AND, OR, NOT(Inverter)]. These are, in principle, all the functions that are neededto build an optical computer. It is therefore anticipated that it willin future be possible to build optical computers which work with lightpulses rather than electrical current and voltage pulses as in the caseof conventional electronic computers. In these supercomputers of thefuture, light pulses will take over the role of electrons as informationcarriers.

Conventional optical information transmission systems, such aslight-guide systems, also operate with light pulses. In light-guidesystems electrical signals are converted into light signals, which passthrough the guide system to the receiver, where they are converted intoelectrical signals or into some other form suitable for the user.

For signal processing in conventional light-guide systems, an opticalsignal is normally converted immediately on reception via anelectro-optical interface into an electrical signal and furtherprocessing is then performed by conventional silicon components.

The optical behavior of some materials, such as LiNbO3, is non-linear,i.e. their various optical parameters exhibit a non-linear dependence onone another. Important types of non-linear functions are opticalpolarization, absorption and refractive index, amplitude modulation ofthe optical intensity, phase modulation, directional changes andfrequency changes.

Non-linear optical components utilize the characteristics of suchnon-linear optical (NLO) materials and are used as electro-opticalinterface between optical and electrical information processing. Theycan amplify incident signals in the same way as transistors or asswitches (or gates in a logic circuit) can control the passage of light.In future generations of computers such phototransistors could well playan important role.

Other examples of non-linear, purely optical components are powerlimiters, oscillators, optical memories, optical sensors and opticalswitches.

An optical switch is described, for example, in W08900714. W08900714discloses a switch matrix with optical non-linear, e.g. bistableelements, lying as optically active layers on a common substratesurface, the substrate surface taking the form of a microstructurecomposed of columns and the optically active layers being applied to theend faces of exposed column ends in a cross-sectional area of columnsand/or on those sides of the substrate remote from the columns.

The principle described has the disadvantage that it takes up arelatively large space, as a result of which, in particular, the localresolution of a switch matrix with such optical non-linear elements islimited. The general technical trend, however, is towards furtherminiaturization from the micro into the nano range.

The object of the invention is to create an optical signal processingdevice operating in the nano range, which comprises a source ofelectromagnetic radiation, a non-linear optical component for theswitching, amplification, limiting and logic operations and means ofdetecting electromagnetic radiation.

According to the invention this object is achieved by an optical signalprocessing device equipped with a source of electromagnetic radiation ofvariable intensity, a non-linear optical component comprising at leastone photoluminescent carbon nanotube, and a means of detectingelectromagnetic radiation.

Carbon nanotubes have unique mechanical and electronic characteristics,which make them suitable for nanomechanical and nanoelectromechanicalapplications, in nanoscalar electronics, for example. To date, however,little has been known about their optical behavior. It has now beensurprisingly found that in addition to electroluminescence carbonnanotubes can also exhibit a pronounced photoluminescence.

The present invention is directed towards the use of carbon nanotubes asnano-scalar purely optical modulators in a non-linear purely opticalcomponent. It utilizes the way in which the intensity of the luminescentlight emitted varies as a non-linear function of the intensity of theelectromagnetic radiation, which is used for excitation purposes.

It has surprisingly also been found that after exceeding a thresholdvalue the intensity of the luminescent light increases approximatelywith the eighth power of the intensity of the exciting electromagneticradiation.

In the device according to the invention, by varying the input intensityof the electromagnetic radiation carried, the output intensity can bedynamically controlled as a function of the input intensity. The signalprocessing is therefore here performed by way of the non-linear purelyoptical component rather than by an electro-optical modulator, so that apurely optical interconnection and hence also purely optical logiccircuits are possible with the very high switching speeds associatedwith these.

According to one embodiment of the invention the non-linear opticalcomponent comprises a substrate and a layer having a number ofphotoluminescent carbon nanotubes.

According to another embodiment of the invention the non-linear opticalcomponent comprises a substrate and a layer having a number ofphotoluminescent carbon nanotubes and also an intermediate layer betweenthe substrate and the layer having a number of photoluminescent carbonnanotubes.

The electromagnetic radiation is preferably monochromatic coherent laserlight.

The invention also relates to a non-linear optical component having atleast one photoluminescent carbon nanotube.

In the non-linear optical component the carbon nanotube may have a thinfilm coating.

In the non-linear optical component the carbon nanotube may also beembedded in a non-oxidizing matrix.

In the non-linear optical component the carbon nanotube may be embeddedin a non-oxidizing matrix, which is transparent for electromagneticradiation.

The carbon nanotube may furthermore be embedded in a non-oxidizing,flexible matrix.

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted.

FIG. 1 shows, by way of example, the spectral distribution of theluminescent light from a specimen of carbon nanotubes when excited by alaser light source with a wavelength of 488 nm.

FIG. 2 shows the non-linear intensity amplification of light by carbonnanotubes.

FIG. 3 shows the threshold values of the intensity amplification forsome multiwall carbon nanotubes produced by microwave Plasma CVD.

FIG. 4 shows the time curve for the decrease in intensity of theluminescent light emitted under various oxygen partial pressures.

An optical signal processing device according to the invention comprisesthe following function groups

-   -   Generation of electromagnetic radiation,    -   Non-linear intensity amplification,    -   Signal reception.    -   The device can be used to perform the following operations:        switching, amplification, limiting and logic operations by means        of optical signals.

The term optical signal is understood to mean an electromagnetic pulsewith a mean wavelength in the ultraviolet, visible or infrared range ofthe electromagnetic spectrum.

In order to illustrate the operating principle of the signal processingdevice with a non-linear optical component, we propose to consider thesimplest structure comprising a laser diode, a non-linear opticalcomponent and a photodiode.

In a device according to the invention, however, any other suitablelight source may also be used as the source of electromagnetic radiationof variable intensity. According to one embodiment of the invention alaser may be used as the source of the electromagnetic radiation ofvariable intensity. According to another embodiment of the invention thevariable intensity is produced by a combination of two conventionallasers. According to a further embodiment of the invention agas-discharge lamp may be used as the source of the electromagneticradiation of variable intensity.

Monochromatic coherent laser light is best suited to the transmissionand processing of information. The semiconductor materials composed ofelements from groups III and V of the periodic table of chemicalelements, such as GaAs, GaAlAs, and InGaAsP have energy gaps, whichpermit the emission of photons in the visible range. Laser diodescomposed of one of these materials can be operated with electricalcurrent as energy source. Photon radiation can also be produced by LEDs.

Laser light with an intensity of between 0.1 and 1500 mW is preferablyused.

In the laser electrical signals are converted into a photon stream,which is processed in the non-linear optical component and forwarded tothe receiver where it is converted back into an electrical signal.

Information is impressed on the laser beam by using an exciting voltageto control the beam intensity, according to a bit pattern, for example.

The electromagnetic radiation incident upon the non-linear opticalcomponent is absorbed by the photoluminescent carbon nanotubes andgenerates, generally with a spectral displacement, photoluminescentlight (FIG. 1), which is finally further processed into a photoelectriccurrent.

The wavelength range used is determined by the nanotube material usedand by its method of manufacture. In the example of embodiment shown inFIG. 1 this is 700±250 nm.

If, in accordance with the invention, carbon nanotubes with anon-linearity of the photoluminescence are used in a non-linear opticalcomponent, this provides a non-linear purely optical component for theaforementioned operations.

The optical non-linear component acts, for example, as a light switch.If the power of a laser beam irradiating such an element increases abovea specific threshold value, i.e. the input intensity for the non-linearcomponent increases, this results in an abrupt increase in the lightemission. The switching control parameter used is therefore the lightintensity P_(in)=σP_(o), which can be obtained, for example, byelectro-optical intensity modulation of the electromagnetic inputradiation. P_(in) is the intensity of the photoluminescent light, σ theamplification factor and P_(o) the intensity of the input light.

This effect allows such optical non-linear components to be used asswitch—elements for purely optical digital data processing.

The degree of optical non-linearity utilizable within the scope of thepresent invention is remarkable. In the case of known non-linearities,the intensity of the signal emitted increases, by the Kerr effect, forexample, proportionally to the cube of the input signal. In the case ofthe “second harmonic generation (SHG)” a square increase in theintensity of the emitted signal can be observed and utilized. Non-linearoptical components with photoluminescent carbon nanotubes can managewith a greatly reduced starting intensity since, as FIG. 2 and 3 show,the intensity of the luminescent light emitted increases with the eighthpower of the input optical pulses.

The threshold value for the non-linear amplification to a certain extentdepends on the method of manufacture of the carbon nanotubes. FIG. 3depicts the intensity curve for multiwall carbon nanotubes produced bymicrowave plasma CVD.

A two-dimensional arrangement of the non-linear optical components is ofparticular interest, for example, in a switch matrix, in which theindividual switch elements have lateral dimensions in the order of 10μm×10 μm and are as closely adjacent as possible.

On the reception side the system contains an optical receiver, whichreceives the optical intensity-modulated signal.

Light-emitting diodes and conventional semiconductor diodes can both beused for receiving signals. The photon beam striking a pn-diode exciteselectrons in the conduction band. At the same time the correspondingnumber of holes is produced in the valence band. When a voltage isapplied a current flows, the strength of which corresponds to theintensity of the incident radiation, and which can be still furtheramplified.

The basic structure of the non-linear optical component according to theinvention may, in principle, comprise a single carbon nanotube. Anembodiment comprising substrate, carbon nanotube layer and anyintermediate layer is preferred. It may be produced by known methods,preferably by deposition from the gaseous phase by a microwave plasma.

In principle, the non-linear optical component may contain carbonnanotubes in any random orientation. The carbon nanotubes are preferablyinserted as a short-walled, regularly deposited layer in order to reducethe light scattering.

The non-linear optical component contains carbon nanotubes. The termnanotubes is generally understood to mean solid, cylindrical discretefibers with dimensions in the nano range. Carbon nanotubes are hollowcarbon fibers having single and multiple wall structures composed of anindividually rolled up graphite layer or concentrically arrangedgraphite cylinders. The graphite layer contains carbon hexagonal ringscondensed to one another all round and is rolled up into a cylindricalshape like a honeycomb so that the carbon hexagonal rings are arrangedhelically.

Inside a layer each carbon atom is cross-linked with three other carbonatoms by sp²-bonds as in graphite, only weak van der Waals forcesexisting from one layer to the other. Such carbon nanotubes havecharacteristics both of a metal and of a semiconductor.

Photoluminescent, single-walled carbon nanotubes may be used in thenon-linear optical component according to the invention. In the contextof the present invention, however, multiwall carbon nanotubes arepreferably used.

Multiwall Carbon Nanotubes (MWCNTs) have a layer structure with anenvelope composed of a number of continuous concentric layers or shellsof sp*2-bonded carbon, which are arranged concentrically around the tubeaxis. An internal cavity may be more or less pronounced. The shells mayhave defects such as holes, bond breaks and included foreign atoms.

The precise structure of the multiwall nanotubes is not critical,provided that they are multilayered and have a structure in which thecarbon atoms within a layer are linked by sp*2-bonds to form hexagonalrings and from one layer to the other by van der Waals forces.

According to one embodiment of the invention the carbon nanotubes aredoped by traces of other elements, in order to influence the opticalcharacteristics.

According to another embodiment of the invention the carbon nanotubesare chemically substituted, in order to influence the opticalcharacteristics.

The carbon nanotubes are preferably inserted as a short-walled,regularly deposited layer in order to reduce the light scattering.

The thickness of the layer containing nanotubes can be adjusted, forexample, by purposely etching back with great precision. Nanotube layerswith a thickness starting from approximately 5 nm can thereby beachieved. They typically have a thickness from 2 nm to 300 nm,preferably 20 to 50 nm.

Methods for the manufacture of carbon nanotubes are known. They are mosteasily manufactured on a large scale by an arc discharge between twocarbon electrodes.

Other known methods consist of laser vaporization and CVD processes, inparticular plasma-based CVD processes.

In the context of the present invention carbon nanotubes which have beendeposited by a microwave plasma-based CVD process are preferably used.

The non-linear optical component according to the invention is suitablyused as ensemble in a matrix with lateral structuring and is producedaccordingly.

In the context of the present invention methods of manufacture in whichthe nanotubes are directly and regularly deposited on a substrate arepreferred for the non-linear optical component.

The manufacture of an oriented array of carbon nanotubes of controlledorientation, diameter, length and shape comprises the following stages:preparation of a substrate, deposition of a catalyst on the substrate,deposition of the nanotubes by thermal separation from a hydrocarbon orby a CVD process on the substrate coated with a catalyst.

According to one embodiment of the invention the substrate istransparent and is composed of quartz, borosilicate or soft glass.

Next a catalyst is applied, which catalyzes the formation of nanotubesfrom a carbonaceous parent material. Such catalysts include, forexample, transition metals, in particular metals from the 8th sub-groupof the Periodic System of Elements (PSE) e.g. iron, cobalt, nickel,ruthenium, rhodium, palladium, osmium, iridium and platinum. Metals ofthe lanthahide and actinide series, and molybdenum are also suitable.

According to one suitable method of manufacture a thin layer of atransition metal composed, for example, of a nickel coatingapproximately 2 nm thick is applied to a substrate such as silicon orglass. The transition metal may also be deposited in the form of smallclusters or single atoms in a wet chemical process.

For the actual manufacture of the carbon nanotubes a carbonaceous parentmaterial and reaction conditions are required, which together with thecatalyst cause the carbon nanotubes to grow from a carbonaceous patentmaterial.

The carbonaceous parent material is usually a hydrocarbon with one toseven carbons, e.g. alkanes, alkenes, aryl groups. Methane, ethane,ethylene, ethyne, acetone, propane and propene are particularlysuitable.

The most important reaction parameter is the temperature. The thermalenergy needed may be supplied in various ways.

The reaction temperature may be between 100 and 1300° C., preferablybetween 300 and 800° C.

If the carbon nanotubes have not already been deposited on a suitablesubstrate, they can be formed by the known methods into a layer or beapplied as a coating to a substrate.

Possible methods of manufacture include both dry coating processes, suchas electrostatic precipitation or electrostatically assisted sputtering,and wet coating processes such as dipping or spraying.

The non-linear optical components can also be arranged as a flatensemble in the form, for example, of a composite film of a polymerresin with regularly arranged nanotubes.

Polymer resins which are suited to the invention include, for example,acrylic resins, polycarbonate, polystyrene, polyester, epoxy resins,polypropylene resins, polyethylene resins, silicone elastomers,thermoplastic polystyrene and polyolefins and polyurethane.

For example, a suspension of the nanotubes in a binder solution,containing acrylic resins, polycarbonate, polystyrene, polyester, epoxyresins, polypropylene resins, polyethylene resins, silicone elastomers,thermoplastic polystyrene and polyolefins and polyurethane in anon-polar solvent such as N,N¹-dimethyl formamide may be applied to asuitable substrate and then dried to form a composite film.

A further embodiment is preferred, comprising the substrate, carbonnanotube layer and a thin film coating, which protects the carbonnanotubes against oxidation. It is also possible to embed the carbonnanotubes in a solid or flexible layer sufficiently transparent for theexciting and the luminescent light, for example in a glass or in aplastic. These compact layers are also capable of protecting the carbonnanotubes against oxidation. If the carbon nanotubes have no protectionor only limited protection against oxidation, the structural changesoccurring under light irradiation at a constant intensity of theirradiated light may lead to a more or less rapid decline over time inthe intensity of the luminescent light emitted (FIG. 4). Since the timecurve for the luminescent light intensity varies as a function of thepartial pressure of oxidizing media in the surroundings of the carbonnanotubes, it may be used as an optical measure for the concentration ofsuch media, for example in an optical sensor.

1. An optical signal processing device equipped with a source ofelectromagnetic radiation of variable intensity, a non-linear opticalcomponent, which comprises at least one photoluminescent carbonnanotube, and with a means of detecting electromagnetic radiation.
 2. Anoptical signal processing device as claimed in claim 1, characterized inthat the non-linear optical component comprises a substrate and a layerhaving a number of photoluminescent carbon nanotubes.
 3. An opticalsignal processing device as claimed in claim 2, characterized in thatthe non-linear optical component further comprises an intermediate layerbetween substrate and the layer having a number of photoluminescentcarbon nanotubes.
 4. An optical signal processing device as claimed inclaim 1, characterized in that the electromagnetic radiation ismonochromatic coherent laser light.
 5. A non-linear optical componenthaving at least one photoluminescent carbon nanotube.
 6. A non-linearoptical component as claimed in claim 5, characterized in that thecarbon nanotube has a thin film coating.
 7. A non-linear opticalcomponent as claimed in claim 5, characterized in that the carbonnanotube is embedded in a non-oxidizing matrix.
 8. A non-linear opticalcomponent as claimed in claim 5, characterized in that the carbonnanotube is embedded in a non-oxidizing matrix, which is transparent forelectromagnetic radiation.
 9. A non-linear optical component as claimedin claim 5, characterized in that the carbon nanotube is embedded in anon-oxidizing, flexible matrix.